fix(scanner): treat colon as plain scalar content in flow mapping keys

[lawrence3699]

Fixes #832

Problem: Parsing {hello:world: [a,b,c]} fails with ',' or '}' must be specified, but this is valid YAML. The unquoted key hello:world is valid because the first : is not followed by whitespace. Without curly braces, parsing succeeds.

Cause: In scanMapDelim(), when inside a flow mapping (startedFlowMapNum > 0), almost every : was treated as a mapping value indicator. The only exception was : followed by / (for URLs). This meant hello:world was split at the first :.

Fix: In flow mapping context, when scanning a plain scalar key (buffer is non-empty), : is only treated as a mapping value indicator when followed by:

• Whitespace (space, tab, newline)
• A flow indicator (,, }, ])
• A flow collection start ([, {)
• A quote (", ')
• End of input

When the buffer is empty (key was already tokenized, e.g. as a quoted string), : remains a delimiter unconditionally. This subsumes the previous :/ special case for URLs.

Before: {hello:world: [a,b,c]} → error
After: {hello:world: [a,b,c]} → map[hello:world:[a b c]]

Validation: All existing tests pass, including the yaml-test-suite (355/402 = 88.3%, unchanged). Five regression tests added covering single and multiple colons in keys, mixed entries, and nested flow collections.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.66%. Comparing base (edee2f9) to head (1a577c7).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #866   +/-   ##
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  Coverage   80.65%   80.66%           
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  Files          22       22           
  Lines        6845     6848    +3     
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+ Hits         5521     5524    +3     
  Misses        905      905           
  Partials      419      419           

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[Copilot]

Pull request overview

This PR fixes flow-mapping key scanning so that : can be treated as plain-scalar content in unquoted keys when it is not followed by a “value-indicating” character sequence, resolving parsing failures like {hello:world: [a,b,c]} (Issue #832).

Changes:

• Updated Scanner.scanMapDelim() to only treat : as a mapping value indicator in flow mappings when the next character makes the delimiter unambiguous (whitespace, flow indicators, collection starts, quotes, or EOS) and the current plain-scalar buffer is non-empty.
• Added regression tests covering single/multiple colons in keys and nested flow structures.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
scanner/scanner.go	Refines : delimiter detection in flow mappings to allow unquoted keys like hello:world while keeping : a delimiter when unambiguous.
decode_test.go	Adds regression coverage for colon-containing unquoted keys in flow mappings, including nested flow collections.

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